This application relates generally to xerographic printing devices including charging devices such as corotrons, scorotrons, AC dicorotrons, AC discorotrons, and the like.
Xerographic printing machines often include charging devices such a corotron, dicorotron, scorotron or discorotron. A corotron is a wire device. A dicorotron is a corotron where the wire has a glass coating. A scorotron is a corotron with a grid on top of it. Similarly, a discorotron is a dicorotron with a grid on top of it. Other charging devices used in xerographic printing machines include pin corotrons and pin scorotrons. The pin variations of these devices substitute a series of pins for a smooth wire or substitute an etched wire having tips resembling a series of pins in a saw tooth shape. Some of these pin based charging devices include an array of pins comprising two or more lines of pins.
Some xerographic printing machines include a photoreceptor. Some photoreceptors are shaped with a surface resembling a belt. When charging the photoreceptor in a xerographic printing machine, it is desirable for the charge to be uniform around the surface of the belt. Variations in the magnitude of the charge around the surface of the photoreceptor are referred to as charge non-uniformities. Charge non-uniformities result in variations in image intensity in a resulting print where the original image does not vary in intensity. Non-uniformities that occur across the width of the photoreceptor are referred to as cross-web non-uniformities. Non-uniformities that occur along the length of the photoreceptor are referred to as down-web non-uniformities. Similar concepts apply to the current uniformity of the charging device.
When operating a scorotron or discorotron charging device, for example, a bias voltage is typically applied. This bias voltage typically corresponds to a charge to which it is desired to charge the photoreceptor. Bias voltages typically range from 300 volts to 1,000 volts. A typical average bias voltage is in the range of 400 to 500 volts.
Some xerographic engines have problems arising from voltage and/or current non-uniformities. Variances in electrical conductivity can be a function of device operation history such as, e.g., powered versus unpowered. This conductivity variation can also cause an operating voltage variation.
Other causes of current and voltage non-uniformities relate to harmful corona effluents in the apparatus and to the method of removing the harmful corona effluents from the machine cavity. The harmful corona effluents are caused by the ionization of the air in the vicinity of a charge that typically exceeds 4,000 volts. This ionization of the air in the vicinity of a high electrical charge generates several gases including ozone. These gases are typically filtered and reconditioned but they can be highly dangerous and even toxic at certain levels of concentration. Therefore, a vacuum is typically employed in the cavity of the machine to remove these unwanted gases including ozone.
Typically, charging devices contain a shield that includes some sort of orifice in order for the vacuum to properly remove the unwanted gases from the machine cavity. However, the quantity, shape and orientation of the orifices in the shield, and the associated air flow generated by the vacuum removal of unwanted gases affect the charge uniformity and the current uniformity of the photoreceptor. Thus, the vacuum removal of unwanted gases from the machine cavity is another among the causes of charge non-uniformity in the photoreceptor.
There are many byproducts of the ionization process described above. In addition to ozone, NOx is another undesirable byproduct. For example, when NOx attaches to H2O, nitric acid is created. Nitric acid is also very harmful and can also be toxic.
In the operation of a xerographic printing device, it is not uncommon for toner to pass through the airflow pattern in and around the zone of the charging device (the volume or area around the wire, the shield and the grid of the charging device). In the process, it is not uncommon for toner suspended in the airflow pattern around the zone of the charging device to be deposited onto the grid.
In the typical fluid mechanics of airflow, a boundary layer forms between the airflow and a stationary surface past which the air is flowing. These boundary layers have a lower airflow rate than the airflow outside of the boundary layer due to the friction created between the flow of air in the boundary layer and the stationary surface past which that air is flowing.
It is not uncommon in the operation of a typical xerographic printing device for the boundary layers that form in the airflow in the zone of the charging device to be clouded with toner debris. This toner debris in the boundary layers of the airflow travels in very close proximity to the photoreceptor. If this cloud of toner debris is not very tightly controlled, the toner escapes from the developing area of the boundary layer.
When the boundary layer interacts with the corona flow and the vacuum flow described above, the interacting airflow effects of the corona flow and the vacuum flow with the boundary layer often disturb the toner particles suspended in the boundary layer. Often, this perturbation of the toner particles suspended in the boundary layer results in the discharge of the toner outside the boundary layer. This discharge of the toner outside the boundary layer often causes the toner to be deposited on the grid in the charging device.
Further, if the cloud of toner suspended in the boundary layer is too weak, it is typical for the flow of the boundary layer itself to disturb the toner in the boundary layer such that pieces of toner are spun or dropped out of the boundary layer. The spinning or dropping of toner particles out of the boundary layer also often results in the deposition of toner particles on the charging device grid in the form of localized dirt build up.
These discontinuities in localized dirt buildup on the grid in turn induce streaks or other unwanted marks in subsequent prints from the xerographic printing device having a charging device with the grid containing the localized dirt buildups. Thus, the faster the localized dirt builds up on the grid, the more rapidly the corresponding print quality from the xerographic printing device having that grid deteriorates, and thus the more frequently the grid needs to be cleaned.
With respect to the architecture of the xerographic printing machine, one convention refers to points furthest inside the machine, that is, points furthest away from a user standing in front of the machine, as inboard portions of the machine. Similarly, according to this convention, portions of the machine closest to the front of the machine, that is, points nearest where a user stands, are referred to as outboard portions of the machine. In one architecture for a xerographic printing machine, the Cross-web orientation of the photoreceptor corresponds to the inboard to outboard or outboard to inboard direction. Similarly, according to this nomenclature, the down-web direction is also referred to as the process direction. This nomenclature is used herein to define a lateral direction and a longitudinal direction.